The objective of this study is to determine whether artificial lights have any adverse biological effects. The effect of three different fluorescent lighting systems (simulated daylight, cool-white, and pink) on the C3H mouse has been studied. The parameters measured include longevity, tumor incidence, reproduction and pathology. The median times for mammary tumor development were 51 weeks, 47 weeks and 42 weeks for female mice kept under the daylight, cool-white and pink fluorescent lights respectively. The first litters of mice kept under the cool-white pink lights were significantly delayed when compared to mice housed under daylight simulating conditions. Finally, the dams in the cool-white group gave birth to smaller litters than did dams exposed to the daylight lights. The possible human health effects of these findings are at the present unclear.